The present invention relates generally to hydroentangled composite nonwoven fabric and is more particularly concerned with a new and improved process for enhancing the cross direction properties of composite fabrics that use a spunbonded web as a base layer and to the new and improved products obtained thereby.
Conventional hydroentangled spunbonded composite fabrics find use as molding substrates, geotextiles and in the medical field as disposable apparel such as surgical gowns and drapes. Hydroentangled fabrics of this type are disclosed in the Suskind et al U.S. Pat. No. 4,808,467 and typically consist of a spunbonded base layer of continuous man-made filaments with one or more overlying cover layers of tissue weight material composed of a blend of wood pulp and synthetic fibers. The tissue weight cover layer is secured to the surface of the base web by hydroentanglement to provide the desired composite structure. Such materials typically have a higher strength in the machine direction than in the cross direction, this lack of squareness being particularly evident in the strip and grab tensile strengths for such materials. The ratio of tensile strengths in the machine direction versus the cross direction (MD/CD) is typically about 1.5:1 and may vary from about 1.3:1 to as high as 4:1.
Material of the type described for use as disposable medical apparel must be cut and arranged so that the strongest fabric direction is oriented to resist directional stresses caused during use by the wearer. Since the rolls of nonwoven fabric are shipped to converters who perform the cutting and sewing operations on automatic equipment, the garment components must always stay oriented with the converting equipment for proper placement in the strongest fabric direction. Consequently, the medical apparel is arranged and cut from rolls of the composite nonwoven fabric so that the strongest fabric direction is always oriented relative to the machine direction of the converting equipment. As can be appreciated, if the fabric possessed improved cross-directional strength characteristics approaching equivalency in both directions, i.e., "square" properties, garment layout and assembly would be significantly easier and less costly to the converter and less critical for wearer protection. Although some spunbonded fabrics can be manufactured to achieve these "square" properties, the manufacturing process must be altered at the time the spunbonded layer is formed, resulting in a much more expensive operation with a resultant drop in fabric productivity.
Spunlaced fabrics have also found use in medical apparel applications. They typically are made as dry-laid webs from staple textile fibers rather than continuous filaments and beneficially exhibit excellent aesthetic and liquid barrier properties but poorer cross-directional strength characteristics and therefore higher MD/CD ratios. The webs are not only fluid repellent and sterilizable but also breathable and comfortable. Examples of such spunlaced fabrics may be found in the Kirayogh et al U.S. Pat. No. 4,442,161 and the Cashaw et al U.S. Pat. No. 4,705,712. The latter patent describes a surface corrugated staple fiber spunlaced fabric having a surface layer of wood pulp that fills the holes in the hydroentangled spunlaced base web material. Before applying the surface layer, the hydroentangled spunlaced fiber web is subjected to a cross direction stretch of 5-80 percent after treating the fabric with a repellent material to lubricate the fabric and make it more easily stretched. While in the stretched and tensioned condition, the fabric is coated with an aqueous slurry of fine fibers, dewatered, and then allowed to contract, resulting in the corrugated composite fabric.
A more recent patent relating to cross-stretched spunlaced composite nonwoven fabric is the Nozaki U.S. Pat. No. 4,883,709. That patent employs a staple fiber base web material that is hydroentangled, resulting in a series of fluid jet traces formed on the layer's surface. The base layer is cross-stretched to provide greater spacing between the fluid jet traces. Shorter fibers are then applied to the stretched base web material in the form of tissue weight sheets and the multilayer structure is subjected to a further water entanglement treatment so that the subsequent water jet traces are more closely spaced from one another than the traces in the stretched base layer. The resultant composite material is said to exhibit greater dimensional stability. However, the tensile strength MD/CD ratio remains at only slightly less than 5:1 and square properties are not obtained by this operation.
The Hagy et al U.S. Pat. No. 4,775,579 teaches a method that involves stretching an elastic meltblown web material and incorporating an absorbent fiber mix by hydroentanglement, while holding the base web in its stretched condition. Following hydroentanglement, the stretched base web is released so that it can return to its original dimensions. The elastic nature of the material makes it well suited for use as an elastic bandage, support or the like. Due to the elastic nature of the filaments, the MD/CD ratio is not significantly altered by the stretching operation.
In accordance with the present invention, it has been found that improved cross direction strength characteristics approaching equivalency in both the machine and cross directions can be achieved when employing a spunbonded web as the base layer of a composite fabric. These beneficial results are achieved by subjecting the spunbonded base web to a cross stretching operation prior to forming the composite fabric.
Accordingly, it is an object of the present invention to provide a new and improved composite spunbonded fabric having enhanced cross-directional properties and a new and improved process for achieving that enhancement. Included in this object is the provision for a composite spunbonded fabric having substantially equal or square strength characteristics in both the machine and cross directions.
Another object of the present invention is to provide a new and improved composite spunbonded fabric of the type described that exhibits barrier and softness properties comparable to spunlaced fabrics while at the same time exhibiting the substantially higher cross-directional strength properties conventionally associated with spunbonded fabrics. Included in this object is the provision for a composite spunbonded fabric having improved dimensional stability coupled with significantly higher strength in the weakest fabric direction, thereby rendering the fabric stronger and more robust for its intended end use. The process for achieving these properties advantageously can be performed in a rapid and facile manner, using a relatively lower total energy input during hydroentanglement, thereby reducing the cost of the resultant composite product.
Other features and advantages of the present invention will be in part obvious and in part pointed out more in detail hereinafter.
These and related advantages are achieved in accordance with the present invention by initially providing a spunbonded base web material consisting essentially of continuous man-made filaments, subjecting the spunbonded base web material to stretching in the cross direction to an extent of at least 5 percent of its original dimension but less than the cross direction elongation of the material under ambient temperature conditions at the time of stretching, stabilizing the base web material in its cross-stretched condition to provide a prestretched base web material substantially free from cross direction tensioning, applying a covering layer of fluid dispersible fibers, preferably in the form of one or more wet-laid wood pulp fibrous webs, to one surface of the relaxed prestretched base web to form a multilayer structure and subjecting the multilayer structure to hydroentanglement while in its relaxed condition to embed the covering fibers in the spunbonded base layer and affix the fiber layer to one surface of the prestretched base material. The resultant hydroentangled nonwoven spunbonded fabric exhibits improved dimensional stability and cross-directional strength characteristics closely approaching those in the machine direction.
A better understanding of the features and advantages of the invention can be obtained from the following detailed description that sets forth illustrative embodiments thereof and is indicative of the way in which the principles of the invention are employed. It is believed that these features and advantages will aid in understanding the process described herein, including the sequence of steps employed and the relation of one or more such steps with respect to each of the others, as well as resulting product possessing the desired features, characteristics, compositions, properties and relation of elements.